I played with the settings a little more.
The minimum text size slider has a range of 18 to 36 on my system. It does not seem to increase the font size of the GSX menu items but it does make the MSFS menu items larger. (Attached 18.jpg and 36.jpg)
For my display (3440x1440), the slider for interface scale has a range of 33 (min) to 100 (max). I have had it set to 43 for a very long time. Earlier, I moved it to about 65 and did not see much change in the GSX menu font size so I thought it wasn't doing anything. I set it to min (33), middle (66), and max (100) and took screen shots. This setting does increase the font size in the menu. However, if I set it to something that makes the GSX menu font large enough, the MSFS text is entirely too large. I'd have to double the size of the ATC window to read it all.
The slider seems to increase the the size of the MSFS menu fonts faster (not sure if you'll understand what I mean by that) than it does the GSX fonts. If I had to guess, I'd say it is just a difference between how the two fonts react in relation to the slider.
Maybe it would be good idea to simply check the display resolution and increase the font size by a couple of points for higher resolution displays?
I'm old... but I'm not blind yet.